The present invention relates generally to a method and apparatus for repairing a defective tube, and more particularly to a method, apparatus, and prefabricated replacement tube for partial tube replacement in a heat exchanger to which access is restricted.
Various types of heat exchangers, such as boilers and waterwalls, are commonly used in hoods and stacks for cooling and/or treating industrial exhaust, for example from steelmaking furnaces, and by utilities (electric companies) to produce electrical energy and/or to cool power generating equipment.
One problem with the operation of heat exchangers is that the heat, gases, dust and substances or fluids to which they are exposed frequently leads to increased rates of corrosion resulting in damage or defects in the tubes of the heat exchanger and subsequent leakage.
A conventional approach to repairing the heat exchanger is to remove a section of the damaged tube, including the defect, over some length, and to install a new replacement tube in its place. The ends of the replacement tube and the stubs of the existing tube are prepared for welding by fitting and beveling surfaces at which they will be joined. Typically, the joining is done manually using shielded metal arc welding (SMAW).
One problem with this approach is that because the weld is performed completely from the outer diameter (OD) of the tube, access is required to all sides of the tube, which is not possible for all heat exchanger designs, particularly boilers such as waterwalls. For example, tubes in a waterwall are frequently connected together along the length of their sides by a metal-webbing or membrane to give added rigidity and strength to the waterwall. Moreover, the waterwall is usually positioned abutting or in close proximity to an outer wall of the boiler (the cold-side). Thus, access is limited to more than half of the outer surface of the tube. This limited accessibility makes it difficult for the welder to achieve good weld tie-in/penetration and often results in less than desirable weld quality and may create problems in the future.
Yet another problem with the above tube replacement method is manual welds performed from the OD are prone to weld defects such as reinforcement or excessive build-up of material on the inner diameter (ID) that lead to restricted fluid flow and accelerated corrosion or erosion at the weld locations.
Another generally known approach for partial replacement of a tube in a heat exchanger, which avoids some of the problems of the above approach, is described, for example, in U.S. Pat. No. 4,047,659, to Vucic (VUCIC). VUCIC discloses accessing the ID of the tube by cutting windows at each end of the replacement tube, the windows intersecting the ends of the replacement tube, and manually welding a portion of the attachment weld from the ID through the window. Covers for the windows are fabricated and welded over the windows from the OD completing the repair.
While a significant improvement over the above approach, this approach is also not wholly satisfactory. A major shortcoming of the approach disclosed in VUCIC is the time required for measuring the section cut from the tube to be repaired, cutting a replacement tube to the correct length, cutting out the windows, preparing the ends of the replacement tube for welding, fabricating covers for the windows and welding the covers over the windows. Another problem with the approach in VUCIC is that all welds are performed manually using a SMAW process. Since the SMAW welds are full penetration, i.e., through the entire thickness of the tube, build-up on the ID for that portion of the welding done from the OD, such as the window covers, can still be a problem. Moreover, because the welds are performed manually weld quality is inconsistent, not-reproducible and can vary from weld to weld.
Accordingly, there is a need for a method and apparatus for repairing a defective tube that provides a weld quality similar to that of a new installation or original fabrication. It is desirable that the method and apparatus eliminate excessive buildup of material from the ID of the repaired tube that can disrupt or reduce fluid flow through the repaired tube and lead to increased erosion/corrosion at the joints. It is also desirable that the method and apparatus enable repairs to be completed quickly with a minimum amount of down time for the heat exchanger. It is further desirable that the method and apparatus be automatic to reduce the level of skilled labor needed.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
It is an object of the present invention to provide a method, apparatus and prefabricated replacement tube for partial tube replacement in a heat exchanger to which access is restricted.
According to one aspect, the present invention provides a method for repairing a defect in a tube using an apparatus including a rotatable torch assembly, a wire feeder for supplying wire to the rotatable torch assembly, and a rotational drive assembly for supporting and rotating the rotatable torch assembly. Generally, the method involves steps of: (i) removing a section of the tube having the defect therein; (ii) fabricating a replacement tube having a wall with an opening extending through a portion thereof, and first and second ends prepared for joining to stubs of the tube formed by removal of the section of the tube having the defect therein; (iii) positioning the replacement tube between the stubs of the tube; (iv) inserting the rotatable torch assembly into the replacement tube through the opening, the rotatable torch assembly configured to align with a joint between the first end of the replacement tube and a stub; and (v) joining the first end of the replacement tube to the stub entirely along a joint therewith by rotating the rotatable torch assembly within the replacement tube. Optionally, slip rings in sliding engagement with the outer surface of the replacement tube hold it in position before it is joined to the stubs. In this embodiment, following the positioning of the replacement tube between the stubs, the slip rings slide up or down to cover the joints. The slip rings have the further advantage of serving as backing rings strengthening the joint when the replacement tube is joined to the stubs, typically by welding.
In one embodiment, the replacement tube is prefabricated having a first end and a second end separated by a predetermined length, and the step of removing a section of the tube having the defect therein involves removing a length of the tube substantially equal to the predetermined length of the prefabricated replacement tube.
In another embodiment, the opening is centrally located between the first and second ends, and the method further includes the steps of: (i) removing the rotatable torch assembly from the opening; (ii) reorienting the rotatable torch assembly; (iii) re-inserting the rotatable torch assembly into the replacement tube through the opening, the rotatable torch assembly configured to align with a joint between the second end of the replacement tube and another one of the stubs; and (iv) joining the second end of the replacement tube to one of the stubs substantially entirely along or around a joint formed there between by rotating the rotatable torch assembly within the replacement tube. The repair of the tube is completed by removing the rotatable torch assembly from the opening, and sealing the opening with a prefabricated cover. Typically, the cover is cover-welded or welded to the opening.
As noted above, the replacement tube is joined to the stubs by welding. Preferably, the replacement tube is joined to the stubs by butt-welding or welding the first and second ends of the replacement tube to the stubs using the rotatable torch assembly. In one version of this embodiment, the apparatus is a Gas Metal Arc Welding (GMAW) apparatus, or a Gas Tungsten Arc Welding (GTAW) apparatus, and the step of welding includes providing a shield gas to the rotatable torch assembly through a rotatable coupling in the rotational drive assembly.
Whichever method for joining is used, preferably, the apparatus further includes a controller for controlling power supplied to the rotatable torch assembly, and operating the rotational drive assembly, and the step of joining the replacement tube to the stubs is performed automatically once the torch is aligned with a joint. The controller controls or adjusts power supplied to the rotatable torch assembly and operates the rotational drive assembly to rotate the torch at a speed that substantially eliminates an excessive build up of material on the inner diameter (ID) of the tube at the joint. Thus, restriction in fluid flow through the repaired tube and corrosion of the joint is reduced.
In another aspect, the present invention is directed to an apparatus for joining a replacement tube to stubs of a tube from which a section of the tube having a defect therein has been removed. The apparatus includes a rotatable torch assembly capable of being inserted into the replacement tube through a window in the replacement tube, to join first and second ends of the replacement tube to the stubs. Wire for joining the replacement tube to the stubs is fed to the rotatable torch assembly by a wire feeder having a wire drive assembly and a wire supply. A rotational drive assembly supports the rotatable torch assembly and rotates it within the replacement tube to join the first and second ends of the replacement tube to the stubs substantially entirely along joints therewith. Generally, the apparatus according further includes a mount to which the rotational drive assembly and the wire feeder are attached, and a latching mechanism for securing the mount to position the rotatable torch assembly within the replacement tube. In one embodiment, the latching mechanism is adapted to be partially inserted into the window to secure the mount to the replacement tube.
In one embodiment, the window in the replacement tube is a centralized window centrally located between the first and the second ends. The rotatable torch assembly is adapted to be inserted into the replacement tube through the centralized window to align with the joint between the first end of the replacement tube and one of the stubs to join the first end to the stub, and to be removed from the replacement tube, reoriented, and reinserted through the centralized window to align with and join the second end to the other stub.
Alternatively, the replacement tube has a number of windows including: (i) a first window located a predetermined distance from the first end of the replacement tube and through which the rotatable torch assembly can be inserted into the replacement tube to join the first end of the replacement tube to one of the stubs; and (ii) a second window located the same predetermined distance from the second end of the replacement tube and through which the rotatable torch assembly can be inserted into the replacement tube to join the second end of the replacement tube to one of the stubs.
In another embodiment, the rotatable torch assembly is adapted to weld the first and second ends of the replacement tube to the stubs. In one version of this embodiment, the apparatus is a gas metal arc welding apparatus (GMAW), and the rotational drive assembly further includes a rotatable coupling through which shield gas is supplied to the rotatable torch assembly. The rotational drive assembly is adapted to rotate the rotatable torch assembly at predetermined rate based on power supplied to the rotatable torch assembly and materials of the wire, the replacement tube and the tube.
In still another embodiment, the apparatus further includes a controller for automatically supplying power to the rotatable torch assembly, and operating the rotational drive assembly. The controller controls or adjusts power supplied to the rotatable torch assembly and operates the rotational drive assembly to rotate the torch at a speed that substantially eliminates a build up of material on the inner diameter (ID) of the tube at the joint. Thus, restriction in fluid flow through the repaired tube and corrosion of the joint is reduced.
In yet another aspect, the present invention is directed to a prefabricated replacement tube for use in repairing a tube having a defect therein. Generally, the replacement tube has a wall with a central opening extending through a portion thereof, and first and second ends separated by a predetermined length. The ends are prepared for joining to stubs of the tube from which a section of the tube having the defect therein has been removed. The central opening is centrally located between the ends, and adapted to enable a rotatable torch to be inserted into the replacement tube through the central opening to join the ends of the replacement tube to the stubs.
Preferably, the central opening is adapted to accommodate the rotatable torch of an automated gas metal arc welding apparatus, which is inserted into the replacement tube to weld the ends thereof to the stubs of the tube substantially without a build up of material at joints. More preferably, the central opening is also adapted to enable an automated welding apparatus to weld a prefabricated cover over the central opening, thereby completing repair of the tube.
Optionally, the prefabricated replacement tube further includes at least one slip ring in sliding engagement with an outer surface of the wall to maintain the replacement tube in alignment with the stub while it is joined to the stubs. The slip ring is adapted to be positioned over the joint between one of the ends of replacement tube and one of the stubs.
Advantages of the method, apparatus and prefabricated replacement tube of the present invention include any one or all of the following:
(i) a weld quality similar to that of a new installation or original fabrication;
(ii) welding is performed from the inner diameter (ID) eliminating excessive buildup or reinforcement typical of outer diameter (OD) welding processes, which can disrupt or reduce fluid flow through the repaired tube and lead to increased erosion/corrosion at the joints;
(iii) weld quality is reproducible and consistent compared to manual applications;
(iv) weld defects associated with manual repair methods are eliminated;
(v) repair is completed entirely from one side of the tubing, for example, fire-side of a waterwall, eliminating access problems associated with welding from both sides; and
(vi) prefabricated replacement tubes having standardized lengths and window configurations can be made available on demand or stocked on site eliminating delays for fabricating replacement tubes.